Field of the Invention
The present invention pertains to the art of microwave cooking and, more particularly, to dynamically controlling the timing of a magnetron in a microwave oven to produce consistent power particularly when the magnetron is cycled at less than 100% power and to compensate for a slow starting magnetron during a short cook cycle.
Discussion of the Prior Art
As widely known in the art, a microwave employs a magnetron to generate microwaves which are directed into an oven cavity for cooking purposes. Microwave ovens that use a switching power supply can effectively lower the power produced by the magnetron by lowering the tube current, but this type of power supply is costly and has reliability issues. Microwave ovens that use a standard cold start power supply operate by cycling the magnetron on and off during a predetermined cycle time to lower the effective power. So, if for example, 50% microwave power is required, the magnetron is turned on 50% of the time, plus some constant factor to compensate for the start time of the magnetron. On the first cycle, the start time is longer than subsequent starts because of tube heating, but this is not normally considered in this type of scheme. This operational scheme can cause inconsistency in the actual power produced from one cook cycle to another. Furthermore, as the tube ages, the start time increases causing less actual power to be produced later in life.
The following chart shows an example of the relative power levels and magnetron timing used with this known type of arrangement, assuming a 2 second average tube start time. The problem is when the tube start time increases or decreases so does the effective power output.
TABLE 1Power onPower offActual powerMW_percent_settingtime [s]time [s]level [%]1052510208222030111930401416405017135060201060702377080264809029190
Based on the above, there is seen to exist a need to produce more consistent power when a magnetron of a microwave oven is cycled at less than 100% power and to compensate for a slow starting magnetron during a short cook cycle.